Centrifugal crushers having a rotary grate have already been well known and, one of them is illustrated in Hungarian Patent No. 160,238 which is incorporated hereinto by reference, as to the essential parts and the environment in which a rotary crusher with a rotating cylindrical grate operates. The crusher illustrated in the above-noted Patent is being used mainly to perform grating or crushing assignments in which the requirement is to produce a strictly defined upper granular size for the grit and which also should have a uniform granular structure.
It is also known that in centrifugal crushers having a rotary grate the calibrating or separation of the grit is performed by the cylindrical grate which is mounted for rotation about the striker-rotary member and, which grate, at the same time, serves also as the impact surface during the grating. This grate performs also the feeding of the material to be grated in a continuous fashion onto the striker-rotary member.
In order to satisfactorily perform the above-noted three major required functions in practical applications, grates consisting of bars turn out to be the best suitable because, in addition to their good material feeding capability, they possess also sufficient strength and rigidity. In addition, the grate bucket made up from the bars can be repaired in a simple manner and the manufacturing of the bars is relatively inexpensive and easy.
The grate bars used heretofore were similar to those used in hammer mills. They are provided with an elongated slot between the bars for purpose of separation of the grit and, the bars of such construction, were able to perform a separation only in a single dimension, namely, according to the thickness of the grit. In the case of hammer mills, or such type of crushers, this was satisfactory because the maximum size of the grit has been considerably smaller than the dimensions of the slot between the bars. However, when it came to the application of such structure to centrifugal crushers employing a rotating grate, it was found that the thickness of a large quantity of the grits will correspond to the dimensions of the slots because the striking bars hit the grits in such a manner that they are impacted onto the grate surface nearly in a vertical fashion, therefore, they can easily pass through the slots of the grate. As a result the grit will contain a large number of flat granules which have a thickness which is satisfactory, however they will have a width which is much greater than the width of the slot. Such granules can not pass through a control sieve having rectangular meshes, therefore, they become classified as excessive in dimension.
One may, therefore, conclude from the above, that the grates which have been constructed with the heretofore known bars, cannot satisfactorily meet stricter calibration requirements.
With such grates a desired upper grit size can be achieved only if the slot between the bars is set to a considerably smaller dimension than the maximum grit size. This would, however, considerably decrease the passing capacity of the grate and also would decrease the uniformity of the grit, because it would lead to an overgrating of the granules. As a result, the quantity of the fractions which are close to the dimensions of the slots would also be decreased due to the presence of the large number of flat granules.